Winchester disk drive systems comprise a vital component for modern computer systems and are used for the storage of large amounts of the data which is accessible at very fast rates. In a Winchester-type disk drive, the magnetization of successive small data cells on a magnetic surface of a disk, allows the storage of in excess of 350 million bytes, or Mega-bytes (350 MBytes) of information in a standard five and one-quarter inch Winchester disk drive. Winchester disk drive systems have a plurality of hard disks made from aluminum and coated with some type of recording media such as magnetic oxide or magnetic metallic material. Each side of the disk may be coated with recording media and each side can store data. Each surface of the disk is associated with its own "read" and "write" head.
The read and write heads are mechanically linked to form a head positioner assembly and the heads move as a single unit across the surface of the disk. Disks are preferably driven by D.C. brushless motors at relatively high speeds in the order of 3600 revolutions per minute (RPM). As the physical size of hard magnetic storage disks drive systems has been reduced, the capacity for storing information has increased. Magnetically stored data is now packed into the disk drive system which must operate with a high reliability factor. For example, no more than 10 uncorrectable errors in 10.sup.13 bits transferred are permitted in the system. The Winchester disk drive system interfaces with a microprocessor controller by the use of large scale integrated (LSI) disk controllers which perform the function of disk drive selection, track selection, sector selection, head loading and error checking. An important part of Winchester disk drive operation is that the disk drive must effectuate data separation and decoding necessary to allow the interface system to synchronize itself to the actual data rate of the disk drive so data may be transferred in an orderly manner to the central processing unit. All Winchester disk drives recover data under certain constraints built into any system. The data rate detected by the data separation circuitry varies from drive to drive due to mechanical factors such as motor speed tolerances. Data is written in a pattern on media through the heads. The media may have defects inherent on its surface. Additionally, the disks spin at a rate which differs slightly from drive to drive. The data recovery patterns must therefore be individually synchronized. In order for a Winchester disk drive system to operate reliably and for the disk controller to transfer data to a central computer in reliable manner, data separation circuitry has operated on phase-locked loop technology. The electronic controller must write a pattern on the Winchester disk drive media and be able to read this pattern back relatively error-free. After a pattern is written into a disk drive on the disk media of the system, the Winchester controller must determined whether the flux reversal that is read is a logical "one" or a logical "zero" In order to do this, the controller must determine where a particular flux reversal occurs relative to others in the media. When a phase-locked loop is used to synchronize the data already written on the disk for use by a central computer, a reference clock signal is generated by the phase-locked loop which is used to determine where each flux reversal is located relative to the other flux patterns. In this sense, the phase-locked loop clock "opens a window", and if the flux reversal is in the window, it is a logical "one"; otherwise the system registers a logical "zero". Although this is at best an extreme simplification of how data is interpreted by a Winchester disk drive controller and central computer, the key factor is to determine whether a given window has recorded a flux reversal or not. The actual position of the flux reversal within the data window is not critical itself. Typically, at the speed and data density with which Winchester disk drives may operate, a data window is only 50 nanoseconds wide.
Within the Winchester disk drive system, the data separation circuitry functions to synchronize the interface circuitry with the data stream coming from the disk. The controller uses the 50 nanosecond window established when the data was recorded and verified by the data separator to reconstruct the data previously recorded on the hard disk drive system. Within the data separation circuity, an analog phase-locked loop may provide the reliability required for accurate data separation. The phase-locked loop constantly analyses the frequency of input real time signals from the hard disk drive and locks a variable oscillator, usually a voltage-controlled oscillator, to that frequency. Using analog phase locked loop techniques, a data separator circuit can be designed so as to achieve a better than plus or minus one nanosecond resolution. The 50 nanosecond window generated by the phase-locked loop of the Winchester disk drive gives the flux reversal signal a maximum of plus-or-minus 25 nanoseconds of shift before the read channel would mistake a "one" for a "zero" or a "zero" for a "one".
Many different factors can cause the flux transition to shift within the data windows. These factors include head asymmetry; phase locked loop tracking error; pulse crowding; and media defects. All these factors have an adverse effect upon the accuracy or precision of the Winchester disk drive in assuring that a flux reversal is placed within the 50 nanosecond wide data window. The cumulative effect of these factors must not be allowed to go beyond the point where the drive can recover data reliably. In order to address the problem of media error, various forms of defect mapping have been undertaken in order to determine the positions on the drive that do not allow recovery of data within the maximum specified hard error rate. As indicated previously, a common maximum hard error rate is 10 bits in error in 10.sup.13 bits read. Conventional defect mapping techniques seek to minimize the mapping time or maximize the effectiveness of the defect map.
The most common methods of defect mapping include:
1. Straight Digital Reads and Writes This method requires a prohibitively long test period. An 85 megabyte (Mbyte) drive would take over 96 hours to achieve a minimally acceptable map.
2. Reduced Data Window with normal "Reads" and "Writes" of Data
Reducing the window width makes a read process much more sensitive to peak shift. This cuts down the time required to make up the location of media defects. Such a test which narrows the size of a window needs to account for extremes in peak shift, and defects may not be detected unless they exceed the window. Reductions in the window width cannot be too great or the drive will give errors everywhere and real defects will be lost in the storm of spurious information.
3. Analog "Missing" and "Extra-pulse" Detection
This technique assumes that most defect areas on the medium which cause excessive peak shifts will have an associated reduction in the analog read amplitude or an extra pulse or both. This assumption is not always true. Some areas of extreme peak shift have very little or no amplitude distortion.
4. Digital Verification of "Missing" and "Extra-pulse" Analog Information
In this method of defect mapping, the analog distortion areas determined as previously described are merely listed as suspect areas. Once the suspect areas have been identified, they can be examined with a severely reduced read" window to determine if there is any gross peak shift. Since there are only a few suspect analog areas, a large amount of time can be spent digitally testing and analyzing these areas. This is the most effective conventional method of defect mapping. Ultimately, the goal of any defect map is to provide a system in which it is both easy to spot hard- to-find defects, and also one in which these defects may be indicated in a relatively short testing period. Previous techniques have included "Phase Margin Analysis" as presented by Memory Technology, Inc., of Santa Clara, Calif., 95050. Memory Technology's Phase Margin Analysis requires making a complex logarithmic plot and re-reading the same track where a given track exhibits a phase shift indicative of a defect. The phase plot can be used to screen out those suspect data readings which represent hard defects and those which might represent defects of a more temporary or soft nature.
A window margin narrowing system has been proposed by Applied Circuit Technology of Anaheim, Calif., 92806. This system uses a shrinking window to determine the location of defects by statistically providing a sample effect window error rate. Both this system and the system of Phase Margin Analysis of Memory Technologies Inc., are relatively complex and require time consuming analysis of data prior to the mapping of defects on the media.
Applied Circuit Technology has proposed a device for measuring time-encoded data pulses written into a hard disk, which allows one to determine whether a drive under test meets required data pulse dispersion limits, which it calls a Time Domain Spectrum Analyzer (TDSA). The Spectrum Analyzer divides the data window into 50 time slots called "buckets." As data pulses are detected, the Analyzer determines at which of the buckets the data pulses fall. As data is read by the Analyzer, each pulse is recorded and pulse counts are accumulated as data read from over 1,000,000 data cells. A histogram may be derived by the Spectrum Analyzer which provides a distribution curve giving information on where within the average cell, the data transition or pulse is occurring. This histogram provides information useful to separate out expected noise from read circuit amplifiers, residual Disk Magnetism, and channel asymmetry; as well as peak and block shift interference, circuit phase shift, bias or cross talk, head positioning and damping. When these defects are factored out, the Analyzer purports to locate media flaws; however, the exact locations of such flaws within particular cells of a track is not provided except to the extent that accumulated data provides guidance.